1. Field of the Invention
The present invention is related to semiconductor devices comprising multiple source-gate-drain combinations, in particular devices that are designed to work at high frequency and high power levels, such as multiple gate-finger High Electron Mobility Transistors (HEMTs), i.e. having at least two gate contacts on the same substrate.
2. Description of the Related Art
High frequency and power devices are generally confronted by a problem of thermal management. In HEMT-devices, heat is generated in a very small area, especially in the channel, close to the gate area. This heat needs to be removed in an effective way.
An established way to produce multiple gate-finger HEMTs is to grow a semiconductor stack (e.g. a GaN/AlGaN stack) on a first substrate, for instance a sapphire substrate, provide ohmic contacts, gate metal and contact metal and a passivation layer, and apply a structure on top of the device, called an ‘airbridge’, to connect the neighbouring contacts, with similar function.
Flip chip bumps are then applied around the HEMT or on a second substrate. The first substrate is then diced to form individual devices which are subsequently attached to the second substrate by flip-chip technology. The air bridges collect the heat generated in the HEMT and remove it laterally. The heat is further removed in vertical direction by the flip-chip bumps. This however results in a device with a considerable height.
In US2003/0040145A1, a method is described for transferring and stacking semiconductor devices. The method allows to produce a single-gate HEMT device by forming it on a first substrate, singulating the first substrate into separate devices, and attaching the individual devices to a second substrate, via a bonding layer. It is not a self-evident step to apply a similar method to multiple gate-finger HEMTs, given that the interconnect between neighbouring contacts would need to be provided in a different way, compared to the air-bridge structures.
US2001/0005043 is related to a semiconductor device and a method of manufacturing the same, wherein a plurality of bipolar transistors are produced on a substrate. The substrate is flipped onto a secondary substrate, and the active areas of the devices are connected through a metal layer applied to the backside of the device, after creation of via holes. The transistors are subsequently diced, to form individual single-gate devices, eg. HEMTs. Given that there is no need in this disclosure for interconnecting corresponding area's (e.g. bases) of the different transistors, to produce a single (e.g. multiple base) device, the problem of avoiding airbridges is not addressed in this document.
Document U.S. Pat. No. 6,214,639 describes a multi-finger HEMT, wherein the source, gate and drain areas are contacted from the backside of the device, by producing through holes to lateral areas which are connected to the multiples sources, drains and gates of the device. However, this technique requires at least one air-bridge to be made. This is shown in FIG. 1a of U.S. Pat. No. 6,214,639, by the dotted lines on the area 14, representing the gate contact area. At these dotted lines, the gate area is crossed by the area's 12. On these locations, air bridges are necessary to separate the contact areas 12 and 14.
Document US2005/0127397 describes a GaN device with heat spreading layer and a thermal via. However, the problem of avoiding airbridges is not addressed here. The heat sinks are moreover not used to interconnect different regions of the device.
One embodiment provides a method for producing semiconductor devices, in particular high frequency and high power devices such as multiple gate-finger HEMTs, which are more compact than the air-bridge structures known in the art, and which are still within the required specifications in terms of heat dissipation.